In recent years, there has been developed a plasma process technique for supplying a high frequency power at a relatively high frequency f and a high frequency power at a relatively low frequency f1 to a reaction chamber. In this plasma process technique, plasma is generated mainly with the high frequency power at frequency f, and the high frequency power at frequency f1 is used to control the behavior of ions in the vicinity of a substrate.
In such a plasma process technique, it is important to accurately measure the voltage, current, power, and the like of frequency f. When f is approximately 1000 times higher than frequency f1, however, the measurement is difficult. This is because the high frequency power at frequency f is modulated by the high frequency power at frequency f1 in the reaction chamber to cause a high frequency power at a frequency f±nf1 (where n is an integer equal to or greater than 0), and a filter circuit that extracts only the high frequency signal at frequency f from the high frequency signal at frequency f±nf1 is not available at present.
Therefore, the voltage, current, power and the like of frequency f±nf1 has conventionally been used as the voltage, current, power, and the like of frequency f. Accordingly, the level variations of the voltage, current, power, and the like of frequency f1 also causes the level variations of the voltage, current, power, and the like of frequency f, thereby leading to a low reproducibility in measurement.